Pneumatic tires for vehicles adapted to be driven in various areas including desert, wet land, waste land, etc. advantageously have high flexibility, a large ground contact area so as to have sufficient load-carrying capacity and traction for running in such areas and an inflation pressure of about 0.1 to 0.3 kg/cm.sup.2 in order to run at a low ground contact pressure. Accordingly cordless tires made only of rubber or equivalently elastic material and capable of retaining a low pressure are preferable as pneumatic tires, especially as tubeless pneumatic tires, for such vehicles.
However, cordless tires are very susceptible to puncuture when the tread strikes nails, glass pieces and other sharp obstacles on the road surface. Particularly tubeless pneumatic tires for vehicles to be driven in various areas are more prone to puncture because of varying driving environments.
While pneumatic tires are adapted to mitigate shocks by the deformation of the tread during travel, shock impact, when acting thereon, deforms the tire and also gives the tire an increased inflation pressure, which acts to reduce the deformation and the absorption of the impact.
The reduction in the volume of the deformed portion of the tire due to the deformation is balanced by bulging deformation of another portion of the sealed interior of the tire, or by an increase in the inflation pressure.
As means for rendering tires less susceptible to puncture, tire cords are incorporated in tubeless pneumatic tires which are made entirely of rubber or equivalently elastic material.
With such tubeless pneumatic tires having cords incorporated therein, the cords extend from one bead of the tire to the other bead, with the opposite ends of the cords substantially secured to the beads. The inflation pressure of the tire acts on the cords and is ultimately supported by the beads.
When the tire cords have a low elongation and extend from bead to bead as fixed thereto, the variation in the interior volume of the tire due to deformation is balanced to a greater extent by the increase of the inflation pressure of the tire than by the bulging deformation of the interior of the tire which is restrained by the cords. This greatly reduces the absorption of impact. Although outstanding in endurance against puncture, the tire does not have flexibility sufficient for use in vehicles for travelling in various areas, rendering such vehicles seriously uncomfortable to ride.
It has therefore been desired to provide tubeless pneumatic tires which have high cushioning ability, sufficient flexibility and outstanding resistance to puncture and which are especially suited for use in vehicles of the type described above. We have already succeeded in developing such tires which have the following improved construction.
The tubeless pneumatic tire of improved construction is made entirely of rubber or equivalently elastic material and has incorporated therein tire cords provided along the inner periphery of the tire and having opposite ends which are not fixedly attached to the beads. While the tire cords give the tread resistance to puncture, an elastic deformable portion composed substantially of rubber or equivalently elastic material is formed between the ends of the cords on each side of the tire and the corresponding bead. With this construction, the rise of inflation pressure is inhibited to a greater extend to afford improved cushioning ability and an extremely comfortable ride.
The tubeless pneumatic tire of such improved construction is produced by a mold comprising at least three components, namely an upper mold element, a lower mold element and a core mold element, defining a cavity having the shape of the tire. However, when rubber or like elastic material is charged into the cavity for molding, the flow of the material acts on the tire cords which are not secured at their ends to the bead portions, consequently disturbing the arrangement of the tire cords and causing movement of cords. Thus difficulties are encountered in accurately positioning the cord ends, with the inevitable result that the tire cords will not be arranged uniformly in the tire obtained.
Although the tire cords are usually placed on the core mold element in accurate position before the cavity is filled with the molding material, the cords, which are arranged in a toroidal annular form in conformity with the shape of the tire inner surface, are difficult to place in position. Additionally, if accurately positioned, the cords will be displaced by the flow of the molding material.